The intra-4f orbitals in rare-earth ions are attractive as a robust material platform for the manipulation of quantum information in a solid because they form deterministic and discrete quantum levels that do not depend on either host material or temperature. We concentrate on erbium oxide (Er2O3) epitaxial thin films, which can match the lattice constant of Si(111) and interact with telecom-band photons. In this study, we investigate the dynamics of excited populations in the intra-4f orbital of Er ions that are indispensable if we are to realize quantum information manipulation.
Figure 1 shows a cross-sectional TEM image of a grown Er2O3 sample and a schematic of a unit cell. The TEM image proves that the Er2O3 is epitaxially grown on the Si(111) surface without crystalline defects . A unit cell of Er2O3 contains 32 erbium ions, 24 at sites with C2 (non-inversion) symmetry and 8 at sites with C3i (inversion) symmetry. Since these energy levels differ at different sites and exhibit narrow inhomogeneous broadening, we can select the excitation site and the level precisely by tuning the laser frequency. Figure 2 shows energy diagrams of the ground [Z(’)1] and 1st excited [Y(’)1-5] states and a color plot of photoluminescence excitation (PLE) spectra. When the energy level of the C2 site is excited resonantly (e.g. Y3 level), the PL is not from the same C2 site, and it is observed only from the spatially separate C3i site (e.g. Y’1-Z’1 transition). This means that energy migrates between distant C2 and C3i sites as a result of a resonant energy transfer arising from a dipole-dipole interaction or wave function overlap. We determined that the time constant of the energy transfer from C2 to C3i is 8 μs by performing a simple rate-equation analysis. This is much shorter than the emission decay from the Y1 to Z1 state (100 μs), but is sufficiently long compared with the time required to complete quantum information manipulation in Er2O3 crystals . These results show that the Er2O3 epitaxial thin layers constitute a promising material platform for the manipulation of quantum information in a solid.
This work was undertaken in collaboration with Hokkaido University and was supported by KAKENHI.